Sunday, 29 April 2012

Just what are these "polymers"?

I’ve already said on my blog that I got into science marvelling about spaceships, stars and planets. I then said that I'm a specialist in polymers. So if I'm a logically consistent human being I must obviously think that polymers are as cool as spaceships. To tell you the truth, without polymers I doubt there would be any spaceships at all, but that's another story. This post is a primer on polymers and a very shallow attempt at showing what is so interesting about them.

A brief look around any vaguely man-made environment and we can quickly see that polymers, in their multitudinous forms, are everywhere in our daily lives. From mobile phones to computers, cars, clothes, carpets, stationery, packaging, paint, and even the very fabric of the human body, polymers are a huge part of everyday life whether we know (or care) or not. So what’s so special about a load of old plastic?

The key is in the architecture. Polymers in their simplest form are chain-like molecules formed of molecular repeat units, monomers. The number of repeat units can range from a few (oligomers) up to several million. The size and flexibility of polymer molecules give them their unique properties.

Here is an example. I take my inspiration here from Grosberg and Khoklov's excellent book Giant Molecules. If one were to make a scaled up version of a polymer molecule, say with monomer units the size of a football (soccer ball), that is a sphere with diameter 20cm. Let’s give our polymer 1 million repeat units. If we rolled our polymer up into a ball, nice and tight, it would look like a giant ball of wool with a diameter of about 20 metres, the size of a large detached house. If it behaved like a real polymer molecule dissolved in a good solvent, e.g. polystyrene dissolved in toluene, it would be an extended random structure around 400 metres in diameter. If we stretched it out end-to-end, it would have a length of 200km. This hierarchy of length scales gives polymers very interesting physical properties. In general, depending the molecular neighbourhood surrounding it, a polymer chains wants to be as disordered as it can possibly be (a manifestation of entropy and the laws of thermodynamics) and this gives it a structure like a messy coil of rope in three dimensions. Pull on the ends of a polymer molecule and you will feel a force resisting you. Straightening the polymer imposes order upon it. The elastic restoring force that pulls an elastic band back to its original shape is just the result of all those individual polymer molecules wanting to be messy coils rather than to be aligned. Add to that the fact that these long, spaghetti-like structures entangle with one another and you have a complex picture of a not-quite-solid, not-quite-liquid.Another interesting property that some polymers possess is glassiness. You may think of a glass as a liquid that is unable to flow. Above the so-called "glass transition temperature" a polymer is molten - its molecules have enough energy to constantly rearrange themselves since the thermal energy all things possess above absolute zero causes molecules to be in perpetual motion, and this energy in this case is enough for the polymer molecules to move past one another (and if they are able, to flow). Cool a polymer below its glass transition temperature though, and the chains no longer have enough energy to move past one another - they are jammed, each chain blocking the motion of its neighbour and so on. This produces a tough material, such as perspex (poly[methyl methacrylate]) which is glassy at room temperature.That's enough hand-wavy polymer physics. Now for some hand-wavy polymer chemistry.

I think its increasingly fair to say that the chemists are really winning the race to drive polymers forward in the 21st century. Polymer chemistry is the incredibly creative art of playing Lego with molecules. In a typical polymerisation reaction, monomers are added together in a simple chain. A polymer chain does not have to be made up of identical monomers though; monomers with different physical properties (size, shape, etc.) and chemical functionalities (electrically charged, acidic, basic, etc.) may be added to build up a polymer with precisely defined properties. Polymers also do not have to be simple chains. Branched polymers are formed by using special monomers like molecular T-pieces, and are limited only by the imagination (and a little bit by chemistry, but that's somebody else's problem). Mindblowingly complex and large structures may be formed even with a single monomer and a single type of branching point. So I hope I have convinced you that chemists are really good at making lots of different types of polymers. The ingenuity and creativity of polymer chemists is truly second to none.So put it all together and what have you got? Polymers by their very nature are complex - in some states disordered and engtangled, in other states glassy and unable to rearrange (not to mention crystalline polymers which are a whole other ball game). In all cases the physics of what is going on a the molecular level is responsible for the macroscopic behaviour of a polymeric material. Couple this with the incredible skills of polymer chemists at building polymers practically to order, with tailored properties and architectures, and we have the key to designing and making an abundance of advanced functional materials. The sky's the limit. I will heartily admit that I have barely scratched the surface of polymer science in this post, not even close. I hope however that you are a little bit wiser about what polymers are and what they do.